1. Field of the Invention
The present invention relates to the acquisition of information, such as pore pressure, from a subsurface formation while drilling. More particularly, the present invention relates to the stabilization and retrieval of apparatuses having utility for acquiring such information.
2. Background of the Related Art
Present day oil well operation and production involves continuous monitoring of various subsurface formation parameters. One aspect of standard formation evaluation is concerned with the parameters of reservoir pressure and the permeability of the reservoir rock formation. Continuous monitoring of parameters such as reservoir pressure and permeability indicate the formation pressure change over a period of time, and is essential to predict the production capacity and lifetime of a subsurface formation. Present day operations typically obtain these parameters through wireline logging via a “formation tester” tool. This type of measurement requires a supplemental “trip”, i.e., removing the drill string from the wellbore, running a formation tester into the wellbore to acquire the formation data and, after retrieving the formation tester, running the drill string back into the wellbore for further drilling. Thus, it is typical for formation parameters, including pressure, to be monitored with wireline formation testing tools, such as those tools described in U.S. Pat. Nos. 3,934,468; 4,860,581; 4,893,505; 4,936,139; and 5,622,223.
Each of the aforementioned patents is therefore limited in that the formation testing tools described therein are only capable of acquiring formation data as long as the wireline tools are disposed in the wellbore and in physical contact with the formation zone of interest. Since “tripping the well” to use such formation testers consumes significant amounts of expensive rig time, it is typically done under circumstances where the formation data is absolutely needed or it is done when tripping of the drill string is done for a drill bit change or for other reasons.
The availability of reservoir formation data on a “real time” basis during well drilling activities is a valuable asset. Real time formation pressure obtained while drilling will allow a drilling engineer or driller to make decisions concerning changes in drilling mud weight and composition as well as penetration parameters at a much earlier time to thus promote safe drilling. The availability of real time reservoir formation data is also desirable to enable precision control of drill bit weight in relation to formation pressure changes and changes in permeability so that the drilling operation can be carried out at its maximum efficiency.
It is desirable therefore to provide an apparatus for well drilling that enables the acquisition of various formation data from a subsurface formation of interest while the drill string with its drill collars, drill bit and other drilling components are present within the well bore, thus eliminating or minimizing the need for tripping the well drilling equipment for the sole purpose of running formation testers into the wellbore for identification of these formation parameters.
More particularly, it is desirable to provide an apparatus that employs an extendable probe for contacting the wellbore wall during a measurement sequence in the midst of drilling the wellbore. The probe is typically positioned inside a portion of the drill string such as a tool collar during normal drilling operation. The section of such a collar that surrounds the probe is an important component of the tool, and its design has an impact on the quality of the measurement, the reliability of the tool and its ability to be used during drilling operations.
The section surrounding the probe, however, is typically not suitable for protecting the probe in its extended position against mechanical damage (cutting, debris, shocks to the wellbore wall, abrasion) and from erosion (from the fluids circulating in the annulus).
It is furthermore well known that the velocity of circulation fluids inside a wellbore has a direct effect on the thickness and integrity of the mud cake (the higher the velocity, the lower the sealing capabilities of the mud cake), which in turn will result in a local increase of the formation pressure near the wellbore wall (also called dynamic supercharging). This effect typically reduces the accuracy of the formation pressure as measured by a probe on a tool. In order to reduce the velocity effects when such a tool is operated and fluids are circulated in the wellbore, it is desirable to increase the flowing area in the annulus, thus reducing fluid velocity near the probe.
Many tools used for taking measurements (wireline and drill string conveyed) employ a pad, piston, or other device that is hydraulically or mechanically extended in association with, or opposite, a probe to make contact with the wellbore wall. Problems arise when there is a failure within the tool or the actuator extending and retracting these devices, leaving the tool deployed or set in the hole. Often times, the retrieval of the tool under such circumstances will permanently damage the hydraulic pistons leaving the tool inoperable or worse, lead to hydraulic leak possibly causing the tool to flood with mud. It is therefore further desirable to incorporate a system in such tools that permits the tools to be withdrawn when faced with such a failure without impacting the operation of the hydraulic and/or mechanical components.